Many off highway trucks have hoist systems designed to lower the body at a high velocity, thereby minimizing the float/lower time, improving the truck's productivity. The highest velocity is limited by the impact the operator is willing to tolerate when the body contacts the pads during the final portion of the float/lower range. In an attempt to maintain reasonable float/lower times, truck designs have compromised on operator comfort allowing harsh impacts of the body on the body resting pads. These harsh impacts, in some cases, have lead to operator complaints. In addition, harsh impacts adversely contribute to overall operator sound levels and undesirable frame loading.
Snubbers have been added to the hoist systems of some off highway trucks to allow the truck to maintain or improve its productivity while, at the same time, reducing the impact when the body contacts the resting pads. Typically, the snubbing functions have been done by reducing the flow area at the cylinder outlet to restrict fluid flow from the cylinder as the cylinder nears its mechanical stop. Traditional cylinder snubbers work by locally increasing the restriction in the head or rod end of each cylinder independently, potentially placing seals and structural parts under extreme pressures. Additionally, many of the in-cylinder snubber designs have been eminently expensive with the added risk of interference between moving parts leading to opportunities for mechanical failures and contamination within the hydraulic system. Moreover, the correct function of the in-cylinder snubbers is dependent on close dimensional tolerances, not only within the cylinder, but also tolerances associated with the cylinder mounting and vehicle frame geometry. Maintaining close tolerances for snubbing components across two cylinder seals is especially difficult for dual stage cylinders typically used on off highway trucks. Finally, the in-cylinder snubbers commonly generate high pressure spikes due to the difficulty of providing precise metering characteristics at the interaction of the moving parts incorporated into a cylinder. Moreover, when a pair of hoist cylinders having in-cylinder snubbing are used, it is extremely difficult to assure that the snubbing loads will be equally shared by both cylinders.
Thus, it would be desirable to provide the snubbing function within the control valve so that the pressure spikes can be more carefully controlled due to the precise metering characteristics of valves and the snub loads will be equally shared between cylinders. It would also be advantageous to provide the snubbing function within the control valve so that the number of components does not have to be duplicated, thereby saving the cost compared with duplicate snubber parts in each cylinder.
The present invention is directed to overcoming one or more of the problems as set forth above.